Contact NEAR MFG, they make the factory FN base rails, they more then likely will have or can make what your looking for.
AT least they did on the ones I have, I'm assuming FNH still uses them as the OEM provider.
My FNH PBR-XP came with the NEAR mount. And Bob A. at FNH sent me a NEAR mount when I first got my PBR 16" that came originially with the 2-peice bases. Which at the time where way out of spec and FNH dropped that un-named mfgr and went with NEAR. Bob A confirmed what I found trying to sight in my 16" PBR, it just would not zero for squat. These two-peice bases where out of spec and Bob sent me a new base free of charge.
Great customer service from FNH.

I could be wrong, but isn't the standand one peice base have 20moa built it?